1. Field of the Invention
The present invention relates to a step motor for use in a driving source of palm-size products such as a camera.
2. Description of the Related Art
In general, a step motor is assembled into small apparatuses such a camera, and there has been a demand for a motor featuring a high efficiency, a low power consumption and small size, because a battery is used for a power source of the compact apparatus. Such a small step motor is disclosed for example in Japanese Patent Application Laid-Open No. 1980-122471. In the conventional step motor, a rotor is disposed in a middle of the motor set, and stators including coils are disposed in opposed relation to the rotor on left and right sides. In the step motor constituted in this manner, a magnet magnetized in four poles is used as the rotor. In a two-phases excitation driving which has been generally performed, a rotation angle per pulse is set 45 degrees. That is, an operation of intermittently repeating rotation in response to a pulse is performed, and a stop state is inserted between rotations. When the step motor having a four-pole magnetization constitution is subjected to the two-phases excitation driving, a non-stable phase and a stable phase alternately exist in the stop state. For the non-stable phase, the stop state is held while the power is supplied to the coils wound around the left and right stators. In other words, the power supply is necessary for maintaining the stop state, hence the phase is referred to as the non-stable phase. On the other hand, for the stable phase, even when the power supply to the coil is cut, the stop state can be held by magnetic equilibrium between the rotor and the surrounding stators.
The step motor having the above-described characteristics is used as a driving source, for example, of a camera lens tube. A feed amount of the lens tube is controlled in accordance with a rotation amount of the step motor, and thereby automatic focusing, zooming, and the like are performed in a constitution. When the feed amount of the lens tube is controlled in accordance with the increment or decrement of the stop position of the step motor, there is some restrictions in the driving control method of the step motor including the non-stable and stable phases in the stop state. That is, in the constitution where the power supply to the step motor is cut after the completion of the feed of the lens tube, only the stop state in which the step motor is magnetically stabilized should be used. In this case, since the non-stable phase cannot be used, it is necessary to select the stable phase, and eventually a stop control is required every two pulses. Conversely, when both the non-stable and stable phases are used in the stop control, the motor can stop at every pulse. Therefore, a resolution of lens feed amount is enhanced. However, the power needs to be supplied to the coil of the stator even after the feed of the lens tube is completed. This increases power consumption.
As means for solving the above-described restriction, a constitution using an interpole is known, and the means is disclosed, for example, in Japanese Utility Model Application Laid-Open No. 1985-141683. The interpole is disposed in opposed relation to the rotor and between the left and right stators, and thereby the stop state in the non-stable phase is magnetically stabilized. That is, the non-stable stop state which could not be stabilized unless the power supply were maintained is magnetically stabilized by the interpole even if the power supply is cut. However, in a conventional stop state stabilization constitution, a single interpole has heretofore been used. Conversely, when a magnetic effect of the interpole is excessively strong, the stop state in the stable phase is adversely influenced and becomes unstable. That is, the presence of the single interpole collapses the magnetic equilibrium between the rotor and the stators in the stable phase. On the other hand, when the magnetic effect of the single interpole is weak, the non-stable phase is insufficiently stabilized during the cutting of the power supply. There is possibility that the stop position eventually fluctuates. As described above, it is difficult to set an optimum magnetic effect of the interpole in the conventional single interpole constitution, and the constitution is insufficient for the stabilization of the stop state of the rotor.
In consideration of the above-described conventional technical problem, an object of the present invention is to further stabilize a stop state of a step motor. To achieve the object, the following means has been developed. That is, according to the present invention, there is provided a step motor comprising a rotor which is rotatable supported and which is comprised of a magnet having magnetic poles magnetized in four angular sections, a first stator which has a pair of counter poles disposed in opposed relation to the magnet and a coil wound around the first stator, a second stator which has another pair of counter poles disposed in opposed relation to the magnet and another coil wound around the second stator, the second stator being opposite to the first stator with respect to the rotor such that the counter poles of the first and second stators surround the rotor to stepwise rotate the rotor in response to electric pulses applied to the coils of the first and second stators, and a magnetic member having a pair of interpoles positioned in a middle of the first and second stators in opposed relation to the rotor such as to stabilize a rest position of the rotor, which is held when the electric pulse is not applied to the coils.
Preferably, said pair of interpoles are spaced apart from each other by an angle corresponding to half of an angular span occupied by one magnetic pole of the magnet. Further, said magnetic member is composed of a magnetic metal material and shaped into a central section and a pair of leg sections extending radially from the central section and electing to define the pair of the interpoles, the central section being registered with a rotation axis of the rotor such that the leg sections can be positioned in opposed relation to the rotor.
According to the present invention, the interpole disposed in opposed relation to the magnet of the rotor has a two-legs construction. In other words, a pair of interpoles are used, not the single interpole. An angle relation of the interpoles defined by two legs is set to half of the angle range occupied by one magnetic pole of the magnet. The interpoles produce a magnetic effect to stabilize either of stable and non-stable phases. Even when no power is supplied to the coil, the motor can stop at the rest position. It is possible to obtain a stop stability superior to that of a conventional step motor. Since the interpole has the two-legs structure, a stop precision of the rotor magnet is stabilized. In either of non-stable and stable phases, the position of the rotor magnet does not deviate and is stabilized, even when the power supply to the stator coil is cut. In the conventional driving method of: cutting the power supply after a rotation operation, the rotor needs to be stopped every two pulses. However, according to the present invention, it is possible to execute the stop control every pulse, so that an operation time can be reduced, or a resolution of the stop position can be enhanced. Otherwise, in the conventional method, the power is supplied to the coil every pulse in order to constantly hold the stop position. However, according to the present invention, since the two-legs constitution is used, the retention of the power supply is unnecessary, and it is possible to remove the power from the coil even in the non-stable phase. Therefore, the power consumption can be reduced.